Jo Y H, Choi W M, Kim D G, Zargaran A, Sohn S S, Kim H S, Lee B J, Kim N J, Lee S
Center for High Entropy Alloys, Pohang University of Science and Technology, Pohang, 790-784, Korea.
Graduate Institute of Ferrous Technology, Pohang University of Science and Technology, Pohang, 790-784, Korea.
Sci Rep. 2019 Feb 27;9(1):2948. doi: 10.1038/s41598-019-39570-y.
We introduce a novel transformation-induced plasticity mechanism, i.e., a martensitic transformation from fcc phase to bcc phase, in medium-entropy alloys (MEAs). A VCrFeCoNi MEA system is designed by thermodynamic calculations in consideration of phase stability between bcc and fcc phases. The resultantly formed bcc martensite favorably contributes to the transformation-induced plasticity, thereby leading to a significant enhancement in both strength and ductility as well as strain hardening. We reveal the microstructural evolutions according to the Co-Ni balance and their contributions to a mechanical response. The Co-Ni balance plays a leading role in phase stability and consequently tunes the cryogenic-temperature strength-ductility balance. The main difference from recently-reported metastable high-entropy dual-phase alloys is the formation of bcc martensite as a daughter phase, which shows significant effects on strain hardening. The hcp phase in the present MEA mostly acts as a nucleation site for the bcc martensite. Our findings demonstrate that the fcc to bcc transformation can be an attractive route to a new MEA design strategy for improving cryogenic strength-ductility.
我们在中熵合金（MEA）中引入了一种新型的相变诱发塑性机制，即从面心立方（fcc）相到体心立方（bcc）相的马氏体相变。考虑到bcc相和fcc相之间的相稳定性，通过热力学计算设计了一种VCrFeCoNi MEA体系。由此形成的bcc马氏体对相变诱发塑性有积极贡献，从而显著提高了强度、延展性以及应变硬化能力。我们揭示了根据Co-Ni平衡的微观结构演变及其对力学响应的贡献。Co-Ni平衡在相稳定性中起主导作用，从而调节低温强度-延展性平衡。与最近报道的亚稳态高熵双相合金的主要区别在于形成了作为子相的bcc马氏体，这对应变硬化有显著影响。本MEA中的六方密排（hcp）相主要作为bcc马氏体的形核位点。我们的研究结果表明，从fcc到bcc的转变可能是一种有吸引力的途径，用于设计新的MEA策略以提高低温强度-延展性。
